Complexity of control of nonvolatile semiconductor storage devices such as a NAND-type flash memory is increasing as the size decreases and higher-density integration is achieved. As a result, a cell current decreases due to reduction of the size of the transistor, and a resistance of a NAND string increases due to increase in the number of word lines per bit line, which affect operation after erasing operation.
Erase sequence performed on a memory cell includes, for example, (1) erase operation for changing threshold values of memory cells to erased state, (2) erase verification operation for confirming that the threshold values of the memory cells became equal to or less than the erased state in the erase operation, (3) soft program operation for solving over-erase of the memory cells in the erase operation, and (4) soft program verification operation for confirming whether the memory cells are over-written back in the soft program operation.
In order to reliably confirm that the soft program is done, so called intelligent soft program verification operation may be executed between the above operations (3) and (4) to confirm whether the memory cells are written back to such an extent that the memory cells do not attain written state as a result of the soft program operation.
However, when the intelligent soft program verification operation is executed, there is a problem in that it takes a longer processing time in the erase sequence.